Germanwings flight 4U 9525 co-pilot Andreas Lubitz hid the details of an existing illness from his employers, German prosecutors say.
They said they found torn-up sick notes in his homes, including one covering the day of the crash.
In their report, Duesseldorf prosecutors did not say what illness Andreas Lubitz had.
German media have said internal aviation authority documents suggested Andreas Lubitz suffered depression and required ongoing assessment.
Prosecutors said there was no evidence of a political or religious motive to his actions, and no suicide note was found.
Andreas Lubitz, 28, and 149 passengers and crew died when Germanwings flight 4U 9525 crashed in the French Alps on March 24.
Data from the plane’s voice recorder suggest Andreas Lubitz purposely started an eight-minute descent into mountains as the pilot was locked out of the cockpit.
In their statement, prosecutors said they seized medical documents from Andreas Lubitz’s two residences – his Duesseldorf flat and his parents’ home north of Frankfurt – which indicated “an existing illness and appropriate medical treatment”.
But “the fact that, among the documents found, there were sick notes – torn-up, current and for the day of the crash – leads to the provisional assessment that the deceased was hiding his illness from his employer”, the report states.
Germanwings, a subsidiary of Lufthansa, refused to comment on the new information, the Associated Press reported.
Earlier on Friday, German media reported that Andreas Lubitz’s notes say he suffered a serious depressive episode when he finished training in 2009.
He went on to receive treatment for a year and a half, the German newspaper Bild reports.
Internal documents quoted by Bild and German broadcaster ARD say a note on Andreas Lubitz’s aviation authority file recommended regular psychological assessment.
Andreas Lubitz’s employers have confirmed that his training was interrupted for several months six years ago, without explaining why.
Lufthansa chief Carsten Spohr has insisted that Andreas Lubitz was only able to resume training after his suitability was “re-established”.
“He passed all the subsequent tests and checks with flying colors,” Carsten Spohr was quoted as saying.
Recovery efforts are continuing at the crash site on the third day following the crash.
Investigators continue to comb the crash site for body parts, debris and the second “black box”, which records flight data and still has not been found.
Family members of some of the passengers and crew who died have visited Seyne-les-Alpes, near the crash site.
They were accompanied by psychologists, paramedics and Red Cross workers, and a youth centre in the town was set up to receive them.
Families are providing DNA samples to allow for identification of victims’ remains.
Many have now left the crash site in the French Alps but more relatives are expected over coming days, including loved ones of a Colombian victim.
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